Funds are requested to support an international workshop to determine the status of the genetic and physical maps of chromosome 21. The workshop, entitled "Mapping Chromosome 21: Present Status and Future Strategies" will be held in Denver on April 2 and 3, 1991 and will include 31 scientists from the United States and 11 scientists from abroad. The primary focus of the meeting will be to assess progress on the physical and genetics map of chromosome 21 since the 1990 workshop. An important aspect will be to assess the progress of the Joint YAC Screening Effort which was established at the 1990 workshop and to make any adjustments felt necessary in that screening effort. An attempt will be made to determine the feasibility and desirability of attempting to completely close the physical map of the chromosome. Assessment will be made of the progress toward a high resolution genetic linkage map using all available published and unpublished data so that strategies can be devised to enhance completion of this objective if necessary. The conference will assess the status of the comparative mouse map of regions of the mouse genome homologous to human chromosome 21, assess available data on isolation of gene sequences from chromosome 21, and determine the advisability of undertaking a major effort toward isolating a large number of chromosome 21 specific gene sequences.